This invention relates to microphones. In particular, this invention relates to directional microphones for use in applications where the microphone is preferably inconspicuous or unobtrusive.
Directional microphones are widely utilized in communications devices for the purpose of increasing signal-to-noise levels and enhancing speech intelligibility. Directional microphones offer discrimination against background noise and undesired acoustic signals originating from directions other than that of the primary receiving lobe of the microphone. As is well known in the art, a first-order directional (or "gradient") microphone element consists of two acoustic input ports used to sense the spatial pressure derivative, dp/dx, of a sound pressure field and produce an output signal proportional to this pressure differential. For unidirectional microphone elements, standard convention defines the "front" entry port to be facing in the direction of maximum sensitivity and the "rear" entry port to be facing in the direction of maximum rejection.
Many applications either require or benefit from flush-mounting or imbedding a microphone in a surface or object. The flush-mounting of an omnidirectional microphone element in a surface is relatively straightforward given the presence of only a single acoustic entry port. For this application, the main design consideration is the pressure enhancing effect of the mounting baffle, which reaches its maximum value of 6 dB (i.e., pressure doubling) at those frequencies for which the baffle size is sufficiently large relative to wavelength. Also well known in the art is the use of acoustic circuits (i.e., cavities and waveguides appropriately dimensioned for a given application bandwidth) for imbedding an omnidirectional element substantially beneath the surface of an object. Such configurations typically call for the consideration and control of waveguide resonances (e.g., the quarter-wavelength resonance of a rigidly terminated waveguide) or perhaps Helmholtz resonances (e.g., those resulting from combination cavity/waveguide input configurations).
In the case of directional microphones, however, flush-mounting or imbedding a microphone element is considerably more challenging for several reasons: 1) the directional microphone requirement for at least two acoustic input ports; 2) the typical locations of front and rear/side entries on commercially available directional microphone elements; 3) the geometry and size limitations imposed by typical application bandwidths; and 4) the critical relative phase and magnitude relationship that must be preserved between the pressure disturbances sensed at each acoustic entry port.
While several imbedded first-order gradient microphone designs have been specifically geared to close-talk telephonic applications, e.g. U.S. Pat. Nos. 4,584,702 to Walker; 4,773,091 to Busche et al.; 4,850,016 to Groves et al., less attention has been given to hands-free applications such as those found in the automotive and computer environments for which the source-to-receiver distance is significantly larger. U.S. Pat. No. 5,627,901 to Josephson et al discloses a first-order gradient microphone imbedded in the center of the upper front edge of a computer monitor intended specifically for hands-free use. This microphone mounting method requires two adjacent orthogonal surfaces and, in lieu of an acoustic circuit, employs a foam-filled cavity with front and rear entry grilles formed into the surface of the monitor. In another notable design, U.S. Pat. No. 5,511,130 to Bartlett et al. has disclosed a second-order gradient microphone consisting of four entry ports and intended for close-talk use in telephone handsets. An unfortunate drawback to the second-order circuit design (relative to a first-order design) is the requirement for front and rear cavities which results in the introduction of Helmholtz resonances due to the interaction of the cavities with the entry ports. In addition, the narrower main lobe and reduced low frequency response (certainly appropriate for close-talk applications in which the proximity effect is inherently present) are not necessarily desirable for hands-free applications.